The ChainMapper class allows to use multiple Mapper classes within a single
Map task.

The Mapper classes are invoked in a chained (or piped) fashion, the output of
the first becomes the input of the second, and so on until the last Mapper,
the output of the last Mapper will be written to the task's output.

The key functionality of this feature is that the Mappers in the chain do not
need to be aware that they are executed in a chain. This enables having
reusable specialized Mappers that can be combined to perform composite
operations within a single task.

Special care has to be taken when creating chains that the key/values output
by a Mapper are valid for the following Mapper in the chain. It is assumed
all Mappers and the Reduce in the chain use matching output and input key and
value classes as no conversion is done by the chaining code.

Using the ChainMapper and the ChainReducer classes is possible to compose
Map/Reduce jobs that look like [MAP+ / REDUCE MAP*]. And
immediate benefit of this pattern is a dramatic reduction in disk IO.

IMPORTANT: There is no need to specify the output key/value classes for the
ChainMapper, this is done by the addMapper for the last mapper in the chain.

The key and values are passed from one element of the chain to the next, by
value. For the added Mapper the configuration given for it,
mapperConf, have precedence over the job's Configuration. This
precedence is in effect when the task is running.

IMPORTANT: There is no need to specify the output key/value classes for the
ChainMapper, this is done by the addMapper for the last mapper in the chain

Parameters:

job - The job.

klass - the Mapper class to add.

inputKeyClass - mapper input key class.

inputValueClass - mapper input value class.

outputKeyClass - mapper output key class.

outputValueClass - mapper output value class.

mapperConf - a configuration for the Mapper class. It is recommended to use a
Configuration without default values using the
Configuration(boolean loadDefaults) constructor with
FALSE.